Cytochemistry coupled with electron microscopy offers the significant advantage of direct visualization of enzyme activities in different cell types by detection of dense reaction products at or near the sites of reactivity. The purpose of the proposed work is to identify ultrastructural enzyme activities associated with developing and differentiated cell membranes and organelles of larval stages of parasitic flatworms (Platyhelminthes: Trematoda) so as to elucidate mechanisms which enable these parasites to infect their molluscan hosts and invade their human hosts. The work will focus on two larval stages of blood flukes (Family Schistosomatidae), the sporocyst and cercaria. The parasites to be studied are of the species Schistosoma japonicum, S. haematobium, and S. mansoni.